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Journal of Oceanography

, Volume 74, Issue 2, pp 169–186 | Cite as

A three-dimensional numerical study of river plume mixing processes in Otsuchi Bay, Japan

  • Kaushik Sasmal
  • Eiji Masunaga
  • Adrean Webb
  • Oliver B. Fringer
  • Edward S. Gross
  • Matthew D. Rayson
  • Hidekatsu Yamazaki
Original Article

Abstract

The three-dimensional numerical model SUNTANS is applied to investigate river plume mixing in Otsuchi Bay, an estuary located along the Sanriku Coast of Iwate, Japan. Results from numerical simulations with different idealized forcing scenarios (barotropic tide, baroclinic tide, and diurnal wind) are compared with field observations to diagnose dominant mixing mechanisms. Under the influence of combined barotropic, baroclinic and wind forcing, the model reproduces observed salinity profiles well and achieves a skill score of 0.94. In addition, the model forced by baroclinic internal tides reproduces observed cold-water intrusions in the bay, and barotropic tidal forcing reproduces observed salt wedge dynamics near the river mouths. Near these river mouths, vertically sheared flows are generated due to the interaction of river discharge and tidal elevations. River plume mixing is quantified using vertical salt flux and reveals that mixing near the vicinity of the river mouth, is primarily generated by the barotropic tidal forcing. A 10 ms−1 strong diurnal breeze compared to a 5 ms−1 weak breeze generates higher mixing in the bay. In contrast to the barotropic forcing, internal tidal (baroclinic) effects are the dominant mixing mechanisms away from the river mouths, particularly in the middle of the bay, where a narrow channel strengthens the flow speed. The mixing structure is horizontally asymmetric, with the middle and northern parts exhibiting stronger mixing than the southern part of the bay. This study identifies several mixing hot-spots within the bay and is of great importance for the coastal aquaculture system.

Keywords

River plume mixing Estuary Barotropic tide Baroclinic tide Diurnal breeze Salt wedge Cold-water intrusion SUNTANS model 

Notes

Acknowledgements

This study was supported by a grant from Tohoku Ecosystem Associated Marine Science (TEAMS), a research program launched by the Ministry of Education, Culture, Sports, Science and Technology (MEXT). The authors wish to express sincere appreciation to the Captains of M. Kurosawa and M. Hirano for their dedicated work in our field surveys. The authors are also grateful to Rusty Holleman who designed the grid generation tool that was used to create the computational domain for this study.

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Copyright information

© The Oceanographic Society of Japan and Springer Japan KK 2017

Authors and Affiliations

  • Kaushik Sasmal
    • 1
  • Eiji Masunaga
    • 2
  • Adrean Webb
    • 3
  • Oliver B. Fringer
    • 4
  • Edward S. Gross
    • 5
  • Matthew D. Rayson
    • 6
  • Hidekatsu Yamazaki
    • 7
  1. 1.Department of Ocean Technology, Policy, and Environment, Graduate School of Frontier SciencesThe University of TokyoKashiwaJapan
  2. 2.Centre for Water Environment StudiesIbaraki UniversityItakoJapan
  3. 3.Disaster Prevention Research InstituteKyoto UniversityKyotoJapan
  4. 4.Department of Civil and Environmental EngineeringStanford UniversityStanfordUSA
  5. 5.Center for Watershed SciencesUniversity of CaliforniaDavisUSA
  6. 6.School of Civil, Environmental and Mining Engineering and the Oceans InstituteUniversity of Western AustraliaCrawleyAustralia
  7. 7.Department of Ocean SciencesTokyo University of Marine Science and TechnologyTokyoJapan

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